The present invention concerns an apparatus and method for aligning a plurality of laser beams, having different wavelengths, along a single optical axis. The invention also concerns a means for dynamically deflecting this superimposed set of beams with a scanning device. The invention also concerns the use of the aligned laser assembly and scanner in displays and display subsystems, including displays that use visible lasers, displays that use UV lasers to induce Stokes fluorescence, upconversion displays that use IR lasers, and combinations thereof.
The alignment and superposition of multiple individual laser beams along a single optical path can provide a number of advantages in a laser subassembly. These advantages include reducing the overall subassembly size; being able to use a single set of optical elements for focusing, modulation, scanning, etc.; and providing a means of delivering multiple lasers simultaneously to a designated target for increased power, multi-color display applications, laser alignment, and others. However, there are significant disadvantages to the presently available systems for accomplishing the optical alignment of multiple lasers along the same optical path. One such prior art system and method for aligning multiple lasers along the same optical path is illustrated in FIG. 1. In FIG. 1 the single optical axis formed from the three separate but aligned lasers, requires the use of two optical elements 10 and 11. During the manufacture of such an optical subassembly, these glass and/or plastic optical elements must be integrated, aligned, and permanently fixed in place. This is not only time consuming but difficult, and tends to have limited long term alignment stability. For example, in addition to the initial alignment problems and the associated cost of the components and labor, the glue used to hold these elements in place is subject to long term thermally induced polymer creep resulting in eventual misalignment. Furthermore, as more elements are added for each additional laser, and the number of refractive or reflective surfaces increases, alignment errors stack up during the manufacturing process.
Typical packaging of an individual laser diode involves soldering of the diode onto an electrically conductive, gold coated heat sink. The diode itself has a back facet known as the High Reflector (HR) and a front facet known as the Output Coupler (OC), both of which are optically coated to reflect the desired emission wavelength of the gain medium's susceptibility curve. A diode shown in FIG. 2 may be soldered onto a small heat sink. Additionally, the device of FIG. 2 may be encased in hermetic packaging to provide additional product life. Each individual packaged laser emits a predominant small set of wavelengths along an optical axis as a function of the output parameters of the laser including temperature and current. This is, by definition, the laser beam which can have multiple spatial modes and frequency components. In FIG. 3, a multiplicity of laser diodes are shown aligned in a row on a single heat sink, with all diodes emitting in the same direction, providing either synchronous or individual addressability via discrete modulation of current to the anodes. Such devices emit light in the same direction but along a plurality of parallel optical axis. In order to direct the output emission from different individually packaged lasers, or from an array of lasers as shown in FIG. 2, so that the light is coincident along the same optical axis, additional components such as prisms, gratings and other optical elements must be integrated into the path. This obviously increases the complexity and the cost of any such system or subassembly.
An object of the present invention is to provide improved multiple laser beam alignment arrangements having not only improved alignment of laser diode outlet beams but also an improved packaging arrangement. Another object of this invention is to provide a single optical beam from a combined set of individual lasers, which can be deflected by a single scanner assembly. Another object of this invention is to provide a laser-scanner subassembly that can be used as a projection display embodying visible lasers, UV lasers, IR lasers and combinations thereof.
It is a further object of this invention to provide a projection display comprising an aligned set of lasers, an optical scanner, and a viewing screen for automobiles, public spaces, advertisements, and other applications.
In accordance with the objects of the present invention, a plurality of optic beams from multiple individual different wavelength lasers are combined without the need for additional prisms, gratings or other combining elements. The combined beams are deflected with a single optical scanner system onto a screen for viewing of displayed information.
According to the present invention, this is accomplished by arranging a plurality of laser diodes one behind the other such that their respective optical axes are coincident. Permanent fixturing of the lasers in this configuration is accomplished by soldering the chips onto the heat sink. No glue is used, thereby eliminating the problems of creep that are associated with polymer adhesives. The light from any one laser propagates through the laser diode chip that is directly in front of it, so long as the devices are positioned within a maximum proximity to each other. Laser diodes of different wavelengths will propagate through the narrow band reflective coatings on the facets of other lasers because the interference layers are very selective. No stimulation of laser action, and only minimal stimulated fluorescence occurs between the sequentially packaged lasers, as the index profile of the devices confines light of multiple wavelengths forcing it to propagate through, and the narrow band facet coatings only allow gain to build up at specific wavelengths. The packaging of lasers directly behind one another for purposes of stimulating laser action between sequentially packaged lasers is known, for example, from a Master-Oscillator Power-Amplifier (MOPA) shown in FIG. 7. A single Fabre-Perot device (oscillator) is packaged directly behind another laser diode (amplifier). The second laser diode is longer than the first in order to provide more gain and is not coated with reflective layers so that it does not have the High Reflector-HR or the Output Coupler (OC) properties of an oscillator. The amplifier in such a device may in fact have anti-reflective coatings to suppress back reflection of the seed emission from the oscillator. Anti-reflection coatings, in conjunction with HR and OC coatings can be applied to increase the through put efficiency of this invention as well. Light from the oscillator of FIG. 4 seeds the amplifier chip, pulling massive gain out of the second device in a single pass (no oscillation). These devices were developed to enable high speed modulation of high power by modulating the low current to the oscillator. The present invention differs from the MOPA architecture in that the output of one laser is not used to stimulate gain of the same wavelength in an amplifier. Instead, the present invention provides a single optical axis for a plurality of different wavelengths.
Deflective scanning of the combined beams may be accomplished by directing them through an acceptable optical scanning system. This architecture could involve several discrete optical elements including lenses, mirrors, and fibers. The use of fibers enables the packaged laser subassembly to be physically removed from the scanner by a greater distance than it could normally be, and it also forces all of the output beams to exit with a circular profile and nearly the same numerical aperture. In addition, the fiber can be used to spatially filter out higher order spatial modes of the beams. The display of information is accomplished by scanning the combined beams and appropriately modulating the current as they traverse through the various pixels on an appropriate screen. A screen can be white or some light color, possibly with light management features integrated to enhance- or directionally control the reflectivity for increased brightness. Additionally the screen can contain a mixture of Stokes or upconversion phosphors, which respond to scanned UV or IR light by emitting visible light. Such upconversion can be a single frequency upconversion or a gated-two frequency upconversion. Gray scale can be achieved by modulating the power output of each laser at every pixel which is done by modulating the drive current. Color mixing is then accomplished by controlling how much of each color is addressed at each pixel. Thus a multi-color RGB projection display can be achieved. Such displays have tremendous utility in automobiles, particularly on dashboards and headrests where curved surfaces exist or where it is difficult or impossible to have electrical wires. Additionally, since laser diodes are small and easy to thermally control using thermistors and Peltier thermal electric coolers, such a display has substantially higher environmental tolerances than existing display technologies for automobiles.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.